HIV-1 avoids destruction by HIV-antigen specific cytotoxic T-lymphocytes (CTL) by decreasing the cell surface expression of human leukocyte antigen (HLA)-A and HLA-B, which are needed to trigger CTL. Natural killer (NK) cells, however, can destroy cells lacking MHC class I molecules since these molecules inhibit NK cell killing. Despite this fact, NK cells are unable to kill HIV-infected cells. Our long-term objective is to determine how HIV prevents NK cells from destroying the infected cells, even though the conditions for the NK cell cytotoxic response appear optimal. One explanation for the inability of NK cells to kill HIV- infected cell is the presence of HLA-C and HLA-E on the infected cell surface. However, HLA-C and HLA-E only prevent a subset of NK cells from killing HIV-infected cells. Another explanation for the lack of NK cell killing is that HIV-1 induces the expression of HLA-G on activated CD4+ T-cells. HLA-G inhibits NK cell cytotoxic responses. The central hypothesis of this proposal is that HIV prevents NK from killing infected cells and controlling viral replication by inducing the expression of HLA-G. To evaluate our objective, we will determine the role of HLA-G cell surface expression, in the context of HIV infection, in disabling NK cell function, and to dissect the required signaling events. This will be accomplished by directing small interfering (si)RNA to HLA-G in HIV-infected cells, and determining the ability of NK cells to become activated and kill HIV-infected cells with decreased HLA-G surface expression. We will also determine the mechanisms by which HIV infection of CD4+ cells induces surface expression of HLA-G. Finally, we will determine the effect of soluble HLA-G secreted by HIV-infected cells on NK cell cytotoxic responses. The overall significance of this study is to provide insight into the mechanism important for allowing HIV to counter cellular immune responses that are differentially regulated by MHC class I molecules.